Dual Gradient Drilling and Riserless Mud Recovery Technology: A Review of Principles, Progress, and Challenges

A comprehensive review of Dual Gradient Drilling (DGD) and Riserless Mud Recovery (RMR) technology was conducted. Deepwater and ultra-deepwater drilling operations were confronted with significant challenges, primarily characterized by narrow formation pressure windows and the physical limitations of conventional riser systems. DGD was proposed as a theoretical framework to address these issues by fundamentally reshaping the wellbore pressure profile. The RMR system, recognized as a mature and commercially successful implementation of the DGD concept, enabled closed-loop recovery of drilling fluid and precise pressure control without employing a conventional riser. This paper systematically reviewed the principles, historical development, global applications, and future challenges associated with this technology. First, the core mechanism of DGD was elucidated, which involved establishing a "virtual wellhead" on the seabed to achieve segmented pressure control, along with the technical architecture and operational workflow of the RMR system. Second, the technological evolution was traced, from its initial conceptualization and subsequent joint industry research and development to its commercialization and expansion into deepwater operations. The review summarized the application outcomes in major global deepwater basins, highlighting its effectiveness in mitigating shallow geohazards, optimizing wellbore architecture, and meeting stringent environmental regulations. Furthermore, the challenges faced during the technology's advancement towards ultra-deepwater environments and intelligent development were analyzed, including issues pertaining to critical equipment reliability, intelligent control systems, adaptability to extreme environments, and cost-effectiveness. Finally, its potential application in emerging fields such as scientific ocean drilling and natural gas hydrate exploitation was explored. This review aimed to provide a systematic reference to support the advancement of deepwater drilling technologies.